Dock levelers or dock boards are commonly used on loading docks to span the gap between the loading dock and the bed of a truck or other carrier parked in front of the dock to enable material handling equipment to conveniently pass between the dock and the truck bed.
The typical dock leveler is mounted within a pit or depression in the loading dock and includes a ramp or deck plate which is hinged at its rear edge to the dock and is movable between a horizontal position, where the ramp is substantially flush with the upper surface of the dock, to an upwardly inclined position.
Hinged to the forward edge of the ramp is a lip that can be moved from a downwardly hanging pendant position to an extended position where the lip forms an extension to the ramp.
After a truck has backed to the loading dock, the ramp is elevated to the upwardly inclined position, while the lip is pendant. The ramp is then lowered and the lip is moved to its extended position and held in that position so that the extended lip will then engage the truck bed as the ramp is lowered back toward the horizontal position. With the lip in engagement with the truck bed, material handling equipment, such as a fork lift truck, can move freely between the dock and the truck bed. After the loading operation is completed and the truck pulls away from the dock, the lip will fall by gravity back to the pendant position.
In the typical dock leveler, the ramp is elevated either by mechanical or hydraulic means. In a mechanically operated dock board, one or more extension springs are mounted in the pit beneath the ramp and are connected to the rear end of the ramp through a lever arm. The force of the springs will pivot the ramp to the upwardly inclined position. The counter balancing springs are designed such that an operator walking outwardly on the ramp, will overcome the force of the springs so that the ramp can be walked down to enable the extended lip to engage the bed of the truck.
In a hydraulically operated dock board, the ramp is moved to its elevated position by actuation of a hydraulic cylinder and by discontinuing operation of the pump in the hydraulic system, pressure in the system will be released so that the ramp will descend, enabling the extended lip to engage the truck bed.
It has also been proposed, as described in U.S. patent application Ser. No. 07/814,002, filed Dec. 26, 1991, now abandoned, to elevate the ramp to the inclined position through use of an inflatable bag. As described in that patent application, a bag is mounted in the pit beneath the ramp with the lower end of the bag engaging the pit floor or supporting frame of the dock leveler while the upper end of the bag is engaged with the undersurface of the ramp. By introducing low pressure air into the bag, the bag will expand, thereby moving the ramp to the upwardly inclined position.